PartitionSizes -> PartitionMetadata

In a following CL, InitPartiitonMetadata not only
takes sizes but also group of each partition. Rename
the struct to a more generic name.

Test: builds
Change-Id: Ie8a7e58b0cccc45e533de368e24f5a73d556de65
diff --git a/common/boot_control_stub.h b/common/boot_control_stub.h
index 65248af..0950997 100644
--- a/common/boot_control_stub.h
+++ b/common/boot_control_stub.h
@@ -45,8 +45,8 @@
   bool MarkSlotUnbootable(BootControlInterface::Slot slot) override;
   bool SetActiveBootSlot(BootControlInterface::Slot slot) override;
   bool MarkBootSuccessfulAsync(base::Callback<void(bool)> callback) override;
-  bool InitPartitionMetadata(Slot slot,
-                             const PartitionSizes& partition_sizes) override;
+  bool InitPartitionMetadata(
+      Slot slot, const PartitionMetadata& partition_metadata) override;
   void Cleanup() override;
 
  private: